Piezoelectric transducer arrangement



April 27, 1965 L. ROSENMAN PIEZOELECTRIC TRANSDUCER ARRANGEMENT Filed July so, 1962 United States Patent 3,181,016 PIEZOELECTRIC TRANSDUCER ARRANGEMENT Leo Rosenman, Van Nuys, Califi, assignor to The Aerospace Corporation, Los Angeles, Calif., a corporation of California Filed July 30, 1962, Ser. No. 213,447 13 Claims. (Cl. 310-83) The present invention relates to electromechanical transducers and, more particularly, to sound wave phenomena using piezoelectric transducer arrangements.

One type of electromechanical transducer is a device which provides an electrical signal in response to a pressure stimulus. These devices may be designed to serve many purposes in many different environments. My invention relates to the class of transducer useful in measuring small dynamic pressures wherein sensitivity, size, frequency response and the like are controlling parameters. Moreover, I prefer to use self generating transducers such as those using piezoelectric elements which require no external source of power.

Mechanically. driven piezoelectric elements are ordinarily displacement sensitive which means that the elements are precharged (polarized) so that the electrical output is proportional to the displacement of the driven portion of the piezoelectric element. The displacement of a resonant system is independent of frequency for a constant force and for frequencies below the mechanical resonance frequency. Thus, piezoelectric transducers will produce an output independent of frequency below resonance and the output will drop rapidly above resonance. In the vicinity of the resonance frequency, the output will depend on the degree of mechanical damping present in the system. It is therefore a common objective of transducer design to provide as high a resonance frequency as possible. This can be achieved by providing a stilf mechanical system but since the output is proportional to the displacement, it may be seen that high resonance frequency and high sensitivity tend to be incompatible. The design objective must therefore be modified to provide as high a resonance frequency as possible consistent with the sensitivity requirement.

A variety of piezoelectric transducers are commercially available, some being suitable for high sensitivity use, such as microphones and others for low sensitivity, such as high pressure instrumentation. However, in hypersonic shock tunnels, the available equipment leaves much to be desired. Piezoelectric microphones with high sensitivity are usually bulky and are not suited for shock tunnel testing. Although some of the piezoelectric transducers designed for high pressures might be dimensionally suitable, they are usually of inadequate sensitivity, because of the high mechanical stiffness required to with stand the high pressures. In addition, both types of prior art transducers suffer from a tendency to respond to the vibrational environment which often makes impossible the obtaining of signal information. For low density shock tunnel measurements the requirement for high sen- I sitivity and small size make it imperative that a high degree of transducer efiiciency be achieved in order to maintain the resonance frequency at a sufficiently high value.

Therefore, a primary object of the present invention is to provide a simple and reliable piezoelectric transducer arrangement suitable for low density shock tunnel environments.

The subject matter which is regarded as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, as to its organization and operation, together 3,181,016 Patented Apr. 27, 1965 understood by reference to the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a simplified schematic drawing exemplifying a conventional construction of an equipment in the art of the present invention;

FIG. 2 is a schematic diagram illustrating one embodiment of of the present invention;

FIG. 3 is a circuit diagram analogy of the present invention;

FIG. 4 is perspective view partially cut away to illustrate another embodiment of the present invention; and

FIG. 5 illustrates an additional embodiment of my invention.

Referring now to the drawing, wherein like numbers refer to similar parts, there is shown in FIG. 1 a cup shaped support housing 10, positionable in an environment having a pressure signal P. Mounted on an exposed surface of the housing 10 is a signal responsive mechanical device such as pressure sensitive diaphragm 12 which is coupled to a cantilevered ceramic piezoelectric crystal arrangement usually referred as a bimorph element 14. An output signal S is obtained from the leads 16 which are coupled to the opposite surfaces of the element 14. The ceramic bimorph element 14 is polarized or charged so that the leads 16 will sense the output signal in proportion to the deflection of the element. Basically the operation is that a pressure signal wave P deflects the diaphragm 12 to bend the element 14. This results in a compression of one surface of the element 14 and a tension in the other so that a positive electrical signal will appear on one of the leads 16 and a negative signal to the other. Opposite deflection of the diaphragm 12 reverses the deformation of the element 14 and the voltage of the signal information of the signal output S However, with this type of construction, wherein the pressure signal P is applied at the end of the element 14, the diameter of the housing 10 must be approximately twice the length of the element 14. Moreover, the mass and compliance of the bimorph result in its being sensitive to vertical vibrations of the housing 10. Changing the mass and compliance to avoid vibrational sensitivity often results in reduced signal sensitivity (S /P) or impracticable stress characteristics. Other constructions of this general nature, such as supporting a bimorph element at both ends and applying the deflection force to the middle, or supporting one end and the center and applying the signal to the other end, do not avoid the problem of vibrational sensitivity.

Referring now to FIG. 2, I have shown the housing It) as supporting the diaphragm 12 in a manner substantially similar to that shown in FIG. 1. However, the arrangement of the parallel piezoelectric beams 18 and 20, in accordance with my invention is substantially different in its operation because of the coupling arrangement illustrated as a bifurcated lever having arms 22 and 23 coupled to the apex 24 of the diaphragm 12 and each coupled to one end of the pair of similary polarized cantilever beams 18 and 20. The beams 18 and 20 are each piezoelectric ceramic bimorph elements. Because of the fragility of most active piezoelectric materials, I prefer that the lever arms 22 and 23 be secured to the free ends of the beams by insulating glue such as a minute drop of epoxy resin as indicated at 25. Using an insulating glue with further objects and advantages thereof, will best be allows the arms 22 and 23 to be fabricated of a metal. Similarly, I usually prefer to use a resilient metal for the diaphragm 12. Moreover, in the present discussion the plane 26 of the diaphragm 12 is defined as the plane perpendicular to the motion of the apex 24 in response to pressure signal P. The deflection of the beam ends is substantially parallel to this plane of the diaphragm. Such operation is accomplished by the beams 18 and 20 being perpendicular to the plane 26 of the diaphragm in C is the compliance of each bimorph; and 7 C is the diaphragm compliance.

the sense that their neutral planes 27 are perpendicular.

In order that the error signals, because of lateral vibrations of the housing lit, may be effectively eliminated, the lead wires 23, 30 and 32 are coupled to the surfaces of the simularly polarized beams 18 and in a cumulative series arrangement so that the signal derived from vectorally similar lateral motion tends to cancel out. However, because of this coupling, opposed lateral motion caused by vertical motion of the diaphragm 12 results in additive output signal S from the cantilever beams 18 and Thus, vibrational noise is substantially inhibited while signal information is enhanced.

In. order that the advantages of the present invention may be clearly understood, 1 have illustrated in FIG. 3 an equivalent circuit diagram. The transformer 36 represents the conversion of fluid pressure to a mechanical force by means of the effective area of the diaphragm I2. The inductance 33 represents the mass of the diaphragm l2, the capacitor 40 represents the compliance of the diaphragm 12, the transformers 42 and 43represent the relative motion of the apex 24 and the ends of the beam 18 and 2%) respectively (which are mathematically equivalent to tan 5:1, being the lever arm drive angle illustrated in FIG. 2); the inductances 45 and 46 representing the bimorph element masses; and the capacitors 47 and 48 representing the element compliances. 'With proper polarization of the bimorph elements 118 and 2t) and the proper coupling of the lead wires 28, 3t} and 32, the signal voltage V will appear in the system and the vibrational voltages V and V are 180 out of phase and will cancel as indicated. The output signal S is obtained across the transformers se and 51 which are equivalent to the electromechanical transducer ratio, With capacitors 53 and 54 being the blocked electrical capacitances of the bimorph elements 18 and 20 respectively. Cursory analysis of FIGS. 2 and 3 indicates that the transformers 42 and 43 provide a l to 1 turns ratio when is 45 .In effect 7 this means that at equals 45, for every millimeter ver-' tical motion of the apex 24 the ends of the beams 18 and id will move in opposite directions and each move a millimeter. T his ratio is changed by altering the lever arms 22 and 23 etc., to change the drive angle The sensitivity of the present invention is the output divided by theinput or simply S /P. Analysis of the circuit of FIG. 3 shows that the signal sensitivity S /P, for

N is the electromechanical transducer ration, A is the diaphragm effective area,

This trigonometricalrelation shows that for maximum signal sensitivity 2 tan 1;) must equal the compliance of bimorphs divided by the compliance of the diaphragms. Thus, it becomes apparent that the lever arm drive angle e is a convenient means for sensitivity control.

Again referring to FIG; 3, analysis of the effect of vibrational forces'may be accomplishedas follows; for vibrations in the flexural plane of the bimorph elements l3 and 2f), the effect will appear as the pair of voltages invention.

V and V inserted in the diagram adjacent to the inductances 45 and 46 representing the masses of the bimorph elements. The magnitude of the voltages V1 and V will be equal to-the mass of the elements times the acceleration of the vibrational stimulus. In accordance with my invention, the polarity of the two voltages 4; mass of the diaphragm. This voltage V will be a tune tion of the mass of the diaphragm times the acceleration V and V is opposed so thatno resultant output will" appearat the output terminals 55. Forvibration along the axes of the diaphragm 12, the effect will appear'as a voltage, V adjacent to the inductance 33 representing'the of the vibrational stimulus. Because of such vibration, a noise output will appear at the terminals 55. *rlowever, this output will be substantially less than that of FIG. 1 where the noise output results from the combined mass of the diaphragm l2 and bimorph element 14.

- Referring now to FIG. 4, I have illustrated another embodiment of my invention wherein the drive arms 22' and 23 are mechanically secured to a pair of piezoelectric bimorph beams 18' and 20 which are supported at their ends by a pair of conductive supporting blocks 56 and 57 with the neutral planes 2? still perpendicular to the plane of the diaphragm 12. ment I prefer to connect the lever arms 22 and 23' to the inner surface of the beams 5.8 and 2G by a conducting glue, such as a conducting epoxy resin, and to polarize the beams oppositely so that the output signal, S is obtained from the outer surfaces of the beams as illustrated by the lead wires 58' and 59 respectively. The grounding 'of the inner surface simplifies the electrical V balancing of the system. This support arrangement is substantially less fragile than that of FIG. 1. However,

the analogies discussed above in connection with FIGS.

2 and 3 still apply, and I am able to obtain maximum signal sensitivity S /P.

A complete microphone transducer arrangement isillustrated in FIG. 5, wherein an aluminum diaphragm i2 is substantially protected by a cap 62 secured to the housing 10. The pressure signal P reaches'the diaphragm l2 through one or more apertures 64 in the cap 62. Also,

insulating clamp 69 which provide compressed contact of resilient hollow stainless steel tubes 70 and conductive copper or brass rods 71. Unlike the signal leadarrangement of FIG. 2, this configuration uses two bimorph elements which are oppositely polarized during fabrication. This permits both inside surfaces to be connected together bythemetal tubes 70 and an outsidebimorphsurface to be grounded through one ofthe metal rods 71 and the support member 68, thus simplifying the construction. The output signal s is coupled to the outer surfaces of the beams'ZZ and 23"through a ground connection 72 and a hot lead 73 of a coaxial cable to transmitthe output 7 signal information S to recording or other equipments.

The electrical coupling of this arrangement is simplified compared to FIGS. 2 and 4. Obviously, many different sensitivities S /P and frequency responses may be de-' signed into components of the configurationillustrated in FIG. 5, whereupon anyone of these designs'may be used as required for a particular operation.

7 While I have shown and described particular embodiments of the present invention, further modifications may occur to those skilled in the art. For instance, the bimorph elements may be coupled in a parallel arrange-- ment, instead of the series arrangements illustrated, if care is taken tobalance the elements electrically to eliminate vibrationalnoise. I desire it understood, therefore, that the appended claims cover all such modifications which do not depart'from the true spirit and scope of my lclaimz 1 An'electromechanical pressure transducerfor deter-' 1 'mining fluid pressure changes inienvironments subject to mechanical vibrational considerations, comprising: p

a cup-shaped housing having an orifice at one end therea diaphragm positioned' to' intercept fluid pressure ,changes'traversing the orifice of said housing;

In this particular embodi- V a pair of bimorph elements securedwithin said housing 3,181,016 6 and having their neutral planes respectively perpentions traversing the orifice, a transducing arrangement dicular to the plane of the diaphragm; comprising: a pair of lever arms coupled respectively to an adjacent a pair of ceramic piezoelectric beams secured within the portion of each of said elements and said diaphragm housing perpendicular to the plane of the diaphragm;

whereby motion of said diaphragm in a direction per- 5 a pair of lever arms coupled respectively between an pendicular to the plane thereof imparts motion to said adjacent drivable portion of each of said beams and elements parallel to the plane of the diaphragm; and the diaphragm whereby motion of the diaphragm in a lead wires coupled to said elements for extracting sigdirection perpendicular to said plane imparts motion nal information, the polarization of said elements to said drivable portion parallel to said plane; and and the connections of said lead wires being such 10 lead wires coupled to said beams for extracting signal that opposing lateral motion of said elements in reinformation, the polarization of said beams and the sponse to deflection of said diaphragm cumulatively connections of said lead wires being such that opposdevelops signal information while vectorally similar ing lateral motion of said beams in response to delateral motion of said elements as a function of an flection of the diaphragm develops signal information vibrational environment, tends to cancel. while vectorally similar lateral motion of said por- 2. An electromechanical pressure transducer for detions, as a function of the vibrational environment, tecting fluid pressure changes in an environment subject result in signals which cancel, said pair of arms each to mechanical vibrational considerations, comprising: defining equal drive angles relative to said plane a cup-shaped housing having an orifice at one end therewhich angle may be varied by design to control f; the signal sensitivity of the transducer.

a diaphragm positioned to intercept fluid pressure changes traversing the orifice of said housing;

a pair of ceramic bimorph elements secured within said housing and having their neutral planes respectively perpendicular to the plane of said diaphragm;

a pair of lever arms coupled respectively between an adjacent portion of each of said elements and said diaphragm whereby motion of said diaphragm in a direction perpendicular to the plane thereof imparts opposed motion to said elements parallel to the plane of the diaphragm, the drive angle ()5 of each of said arms being equal whereby each of said elements responds a substantially identical magnitude to motion of said diaphragm; and

lead wires coupled to said elements for extracting signal information, the polarization of said elements and the connections of said lead wires being such that op- 5. In an electromechanical pressure transducer for detecting fluid pressure changes in an environment subject to mechanical vibrational considerations having a housing defining an orifice and a diaphragm positioned to intercept 25 fluid pressure changes traversing the orifice, a transducing arrangement comprising:

a pair of bimorph elements secured with their neutral planes respectively perpendicular to the plane of the diaphragm;

a pair of lever arms coupled respectively between an adjacent portion of each of said elements and the diaphragm whereby pressure fluctuation induced motion of the diaphragm in a direction perpendicular to the plane thereof imparts opposed lateral motion in said elements parallel to the plane of the diaphragm, the drive angle 4) of each of said armsbeing equal whereby each of said elements responds a posing lateral motion of said elements in response to deflection of said diaphragm cumulatively develops signal information while vectorally similar lateral motion of said elements as a function of the vibrational environment, cancels.

substantially identical magnitude to motion of said diaphragm; and

means coupled to said elements for extracting signal information S so that opposing lateral motion of said elements in response to a pressure signal P cumumining fluid pressure changes in environments subject to latively develops signal information while vectorally similar lateral motion, as a function of the vibrational environment, cancels whereby signal sensitivity S /P is of function of llll gb.

6. In an electromechanical pressure transducer for developing an output signal S in response to a fluid pressure signal P within an environment subject to 'mechanical vibrational considerations, the transducer having a housing defining an orifice and a diaphragm positioned to intercept fluid pressure changes traversing the orifice, a transducing arrangement comprising:

a pair of bimorph elements secured with their neutral planes respectively perpendicular to the plane of the diaphragm;

a bifurcated lever coupling at'drive angles an adjacent portion of each of said elements to the dia-' phragm, the drive angle of each of said arms being equal so that each of said elements responds a sub- 3. An electromechanical pressure transducer for detervibrational considerations, comprising:

a cup-shaped housing having an orifice at one end therea diaphragm positioned to intercept fluid pressure 7 changes traversing the orifice;

a pair of ceramic piezoelectric beams secured within said housing perpendicular to the plane of said diaphragm;

a pair of lever arms coupled respectively to the ad acent free end of each of said beams and said diaphragm whereby motion of said diaphragm in a direction perpendicular to said plane imparts motion to the ends of said beams parallel to said plane; and

lead wires coupled to said beams for extracting signal information, the polarization of said beams and the connections of said lead wires being such that opposing lateral motion of the ends of said beams In response to deflection of said diaphragm develops stantially identical magnitude to the pressure Signal P induced motion of said diaphragm, whereby signal sensitivity S /P is of function of tan qb; and

means coupled to said elements for extracting an output signal S in response to pressure signal P while cancelling noise developed by similar lateral motion of said elements as a function of the vibrational environment.

cumulative signal information while vectorally similar lateral motion of said beams, as a function of the I -vibrational environment, tends to cancel, said pair of arms each defining a drive angle relative to said plane which angle may be varied to control the signal sensitivity of the transducer.

4. In an electromechanical pressure transducer for determining fluid pressure changes in environments subject to mechanical vibrational considerations, and of the type using a housing having an orifice at one end thereof and a diaphragm positioned to intercept fluid pressure fluctua- 7 termining fluid pressure changes in environments subject to mechanical vibrational considerations and of the type having a diaphragm positioned to intercept fluid pressures fluctuations, a transducing arrangement comprising: a pair of oppositely polarized parallel ceramic bimorph elements secured with a free end of each being ad- 7. In an electromechanical pressure transducer for dejacent to the diaphragm, said pair of elements having a pair of inner surfaces and a pair of outer sur-' faces;

a bifurcated lever coupled respectively between said free 1 ject to mechanical vibrational considerations and of the type using a housing having a diaphragm positioned to intercept fluid pressure fluctuations, a transducing arrangement, comprising: 7

a pair of oppositely polarized ceramic bimorph elements resiliently secured Within the housing and having their major axes perpendicular to the plane of the diaphragm;

a bifurcated lever in a plane perpendicular to the neutral planes of said pair respectively for coupling the adjacent drivable end portion of each of said pair to the diaphragm whereby pressure signal responsive motion of the diaphragm imparts perpendicular and opposed motion to said portions;

resilient spacer means electrically coupling the inner surfaces of said pair of a point remote from said portions; a

a'lead wire coupled to one outer surface to said pair for extracting signal information; and

grounded circuit means coupled to the other outer surfaces'of said pair whereby opposing lateral motion of said portions develops signal information.

9. In an electromechanical transducer for determining signal variations in environments subject to mechanical V vibrational considerations and of the type having a mechanical device positioned to intercept 'signal variations, a transducing arrangement comprising:

a pair ofoppositelypolarized parallel ceramic bimorph V cantilevered elements each having one end secured and itsother end being free and being adjacent to the said'mechanical device, said pair of bimor-ph elements having a pair of inner surfaces and a pair of outer surfaces;

a bifurcated lever coupled respectively between the free ends of the'bimorph elements and the device so that signal induced motion of the device imparts equal and opposite lateral motion to said'free ends in a direction perpendicular to the neutral planes to efiect I flexing thereof; i

one pair of said element. surfaces being electrically coupled together; and V lead wires coupled to the other pair of said element surfaces for extracting signal information.

' 10. In an electromechanical transducer for determining signal variations in environments subject to mechanical vibrational considerations and of the type having a mechanical'device positioned to intercept signal variations, a transdueing arrangement comprising: 7

a pair of oppositely polarized ceramic bimorph elements resiliently secured to have. their major axes parallel to the motion of the mechanical device in response to signal variations; 7 a bifurcated lever having divergent axes in a plane perpendicularto the neutral planes of saidpair of bimorph elements respectively and coupling the adjacent drivable end portion of each of said pair to the device so that signal responsive motion of the device imparts joppositela teral motion to said end portions;

resilientj spacer means coupling the inner surfaces'of a lead wire coupled to one outer surface of said pair for extracting signal information; and V grounded circuit means coupled to the other outer surface of said pair whereby opposite lateral motion'of said pair develops signal information between said lead wire and said grounded circuit means.

11. In an electromechanical transducer for determining signal variations in environments subject to mechanical vibrational considerations and of the type having a mechanical device positioned to intercept signal variations, a transducing arrangement comprising; I

a pair of similarly polarized parallel ceramic bimorph elements each having an inner and an outer ceramic surface and each secured with a free end adjacent to the mechanical device;

a bifurcated lever coupled respectively between the said freeends and the device'so that signal induced motion of the device imparts equal and opposite lateral motion to said free ends in a direction perpendicular to their neutral planes;

a lead shunt coupling the outer surface of one of said elements to inner surface of the other said element;

and I 1 a pair of lead wires coupled to the other of said sur-.

to the motion of the mechanical device when'respending tosignal variations; a bifurcated lever mechanically coupling the adjacent drivable end' portion of each of said pair of bimorph elements to the mechanical device so that signal responsive motion of the device imparts equal and opposite lateral motion to said end portions in a j direction perpendicular to the neutral plane of each of said pair;

resilient spacer means electrically coupling the inner surfaces of said pair at a point remote from said'end portions; and t circuit means coupled to the outer surfaces of said pair for extracting signal information. :13, In an "electromechanical transducer ing signalvariations in environrnentssubject to mechanical vibrational considerations and wherein a mechanical device is positioned to intercept signal variations, ajtrans 'ducing arrangement comprising: V V

a pair of, oppositely polarized ceramic-bimorph ele-f ments positioned to have their major axes parallel to the motion of the mechanical device when responding to signal variations; a bifurcated; lever mechanically coupling the adjacent drivable end portion of each of said pairof bimorph' elements to the mechanical device so that signal responsrve motion of the device imparts equal 'and opposite lateral motionto said-end portions in a direction perpendicular to the neutral plane of each i of said pair; Y j means for resiliently supporting said pair at apoint remote from said end portions; and i electric circuit means coupled to the surfaces of said pair for extracting signal information and inhibiting vibrational noise signals. i

References Cited by theExarniner UNITED STATES. PATENTS r MILTON T HIRSHFIELD, art/nab Examiner;

for determin- V. V 

1. AN ELECTROMECHANICAL PRESSURE TRANSDUCER DETERMINING FLUID PRESSURE CHANGES IN ENVIRONMENTS SUBJECT TO MECHANICAL VIBRATIONAL CONSIDERATIONS, COMPRISING: A CUP-SHAPED HOUSING HAVINGG AN ORIFICE AT ONE END THEREOF; A DIAPHRAGM POSITIONED TO INTERCEPT FLUID PRESSURE CHANGES TRAVERSING THE ORIFICE OF SAID HOUSING; A PAIR OF BIMORPH ELEMENTS SECURED WITHIN SAID HOUSING AND HAVING THEIR NEUTRAL PLANES RESPECTIELY PERPENDICULAR TO THE PLANE OF THE DIAPHRAGM; A PAIR OF LEVER ARMS COUPLED RESPECTIVELY TO AN ADJACENT PORTION OF EACH OF SAID ELEMENTS AND SAID DIAPHRAGM WHEREBY MOTION OF SAID DIAPHRAGM IN A DIRECTION PERPENDICULAR TO THE PLANE THEREOF IMPARTS MOTION TO SAID ELEMENTS PARALLEL TO THE PLANE OF THE DIAPHRAGM; AND LEAD WIRES COUPLED TO SAID ELEMENTS FOR EXTRACTING SIGNAL INFORMATION, THE POLARIZATION OF SAID ELEMENTS AND THE CONNECTIONS OF SAID LEAD WIRES BEING SUCH THAT OPPOSING LATERAL MOTION OF SAID ELEMENTS IN RESPONSE TO DEFLECTION OF SAID DIAPHRAGM CUMULATIVELY DEVELOPS SIGNAL INFORMATION WHILE VECTORALLY SIMILAR LATERAL MOTION OF SAID ELEMENTS AS A FUNCTION OF AN VIBRATIONAL ENVIRONMENT, TENDS TO CANCEL. 